1. Field of the Invention
The present invention relates to asphaltic compositions having viscoelastic properties that are suitable for use as binders in dense graded and open graded hot mix asphalt pavement. The compositions comprise blends of asphalt or bitumen and synthetic polymers which are storage stable between the time of preparation and the time of use in hot mix pavement. The present invention also relates to methods of preparing and methods of using the compositions and the products produced by the process of the present invention.
2. Discussion of Related Art
Asphalt has certain physical properties that vary widely with changes in temperature. For this reason, polymers often are added to asphalt to extend the range of its physical properties. Polymers can reduce the tendency of the asphalt to creep and rut in warm weather by increasing its high temperature viscoelastic stiffness; typical paving grade asphalts have softening points below 50.degree. C., while softening points greater than 55.degree. C. are desired for resistance to creep in hot weather; and polymers can minimize cracking and stripping in cold weather by improving the asphalts low temperature viscoelastic properties.
Not all asphalts and polymers form compatible mixtures having storage stability. The storage stability of polymer modified asphalt is greatly affected by the compatibility of the asphalt with the particular polymer. Incompatible and partially compatible mixtures easily undergo phase separation.
Furthermore, some polymer modified asphalts are suitable for roofing applications but cannot be used in hot mix paving applications where the blend must be sufficiently fluid at mixing temperature to adequately coat the aggregate particles. For example, an asphalt binder for dense graded hot mix paving should have a viscosity at 135.degree. C. between 150 and 2000 cPs, and a binder for open graded hot mix should have a viscosity at 135.degree. C. between 3000 and 8000 cPs.
Various methods have been suggested for making polymer-modified asphalt compositions that are sufficiently compatible to be acceptably storage stable and that also have viscosities and softening points in the range required for a particular type of application. Some, for example, rely on selection of the appropriate polymer and other additives and require a specific sequence of blending and reaction steps to effect the final properties of the asphalt-polymer mixture (see, for example, U.S. Pat. No. 4,600,635 for use of a bitumen-based adhesive composition comprising bitumen, oil, and an ionomeric elastomer (such as sulfonated, carboxylated or phosphonated EPDM or butyl rubber) in less than 15% weight, preferably 3%-8%, that is neutralized by metal cations; British Patent No. 1,534,183 discloses inter alia use of a metal alcoholate and the addition of from 5% to 95 % wt. of an ethylene/vinyl ester copolymer and bitumen partially crosslinked by means of --O--M--O--, --O--B--O-- or --O--CRR'--O-- bridges; or U.S. Pat. No. 4,882,373 for acid modification of asphalt with subsequent contact with an oxygen-containing gas, and mixing with a thermoplastic elastomer, and an unsaturated functional monomer.)
These examples require specific chemicals and/or processing steps not required by the present invention.
U.S. Pat. No. 4,371,641 teaches that a polymer, preferably an unsaturated elastomer, e.g., EPDM or butyl rubber, or a thermoplastic polymer (from 5-25 wt %) and an appropriate filler can be added to airblown asphalt to make it suitable for use as a roofing material by increasing the asphalt's stability towards oxidation and decreasing its sensitivity to ultraviolet radiation. The thermoplastic or elastomeric polymer must contain anionic groups such as carboxylate, phosphonate or, preferably, sulfonate groups which can be neutralized by addition of a salt of a carboxylic acid, preferably a metal acetate, more preferably zinc acetate. Applicants have found that the weight percents of polymer specified in this disclosure produce blends that are too viscous to coat the aggregate in hot mix paving materials.
Accordingly, none of the foregoing references teaches nor suggests the road paving asphaltic compositions having the storage stability (as evidenced by appropriate softening points of the blends and by lack of phase segregation between the asphalt and polymer phases) and viscoelastic properties over a wide range of temperatures required to produce a superior binder for dense graded and open graded hot mix asphalt for road paving applications that is disclosed by the applicants, nor does any teach nor suggest the process of preparing or using applicants' composition.